Magnetic recording medium

ABSTRACT

A magnetic recording medium is disclosed, comprising a support having provided thereon a substantially nonmagnetic lower layer by coating a lower layer coating solution comprising a nonmagnetic powder dispersed in a binder and drying, and a layer having a thickness of from 0.01 to 0.15 μm by coating a magnetic layer coating solution comprising a ferro magnetic magnetic metal powder or a hexagonal ferrite powder dispersed in a binder, wherein the magnetic layer contains diamond particles having an average particle size of from ⅕ to 2 times the thickness of the magnetic layer in an amount of from 0.1 to 5.0 mass % based on the ferromagnetic metal powder.

FIELD OF THE INVENTION

[0001] The present invention relates to a coating-type magneticrecordingmedium (i.e., a magnetic recording particulate medium), inparticular, to a magnetic recording medium having a thin magnetic layerfor high density recording.

BACKGROUND OF THE INVENTION

[0002] Along with a sudden increase in the amount of data to be dealtwith in the field of magnetic disc, the increase of the capacity offloppy discs has been demanded.

[0003] Further, in the field of magnetic tapes also, with the spread ofoffice computers, such as minicomputers, personal computers and workstations, magnetic tapes for recording computer data as external storagemedia (so-called backup tapes) have been eagerly studied in recentyears. For putting magnetic tapes for such usage to practical use, theimprovement of recording capacity has been strongly demanded conjointlywith the miniaturization of a computer and the increase of informationprocessing ability (e.g., information throughput).

[0004] Magnetic heads working with electromagnetic induction as theprinciple of operation (an induction type magnetic head) areconventionally used and spread. However, magnetic heads of this type areapproaching their limit for use in the field of higher density recordingand reproduction. That is, it is necessary to increase the number ofwinding of the coil of a reproduction head to obtain larger reproductionoutput, but when the winding number is increased, the inductanceincreases and the resistance at high frequency heightens, as a result,the reproduction output lowers. In recent years, reproduction headswhich work with MR (magneto-resistance) as the principle of operationare proposed and come to be used in hard discs. As compared with theinduction type magnetic disc, several times of reproduction output canbe obtained by the MR head. Further, since an induction coil is not usedin the MR head, noises generated from instruments, e.g., impedancenoises, are largely reduced, therefore, it becomes possible to obtain agreat S/N ratio by lowering the noise coming from magnetic recordingmedia. In other words, good recording and reproduction can be done andhigh density recording characteristics can be drastically improved bylessening the noise of magnetic recording media hiding behind theinstruments.

[0005] There is disclosed in JP-A-10-340445 (the term “JP-A” as usedherein means an “unexamined published Japanese patent application”) amagnetic recording medium having an upper magnetic layer which showsexcellent durability, electromagnetic characteristics, in particular,excellent output and overwriting characteristics by setting thethickness of an upper magnetic layer at 0.5 μm or less to suppress thefluctuation at interface between a magnetic layer and a nonmagneticlayer. For suppressing interfacial fluctuation, the same patent adoptswet-on-dry coating of coating a nonmagnetic lower layer and then coatingan upper magnetic layer after the nonmagnetic lower layer has beendried. However, the thickness of 0.3 μm of a magnetic layer prescribedin the patent is too thick and is not suitable for higher densityrecording. Further, in general, the durability of a magnetic recordingmedium produced by wet-on-dry coating is liable to be deteriorated, andthis tendency is more conspicuous when the thickness of a magnetic layeris thin.

[0006] JP-A-5-298653 ensures high electromagnetic characteristics andhigh durability by setting the average thickness of an upper magneticlayer at 0.01 to 0.3 μm to control the fluctuation at interface betweena magnetic layer and a nonmagnetic layer so that (standard deviation ofa magnetic layer thickness)/(average magnetic layer thickness) becomeswithin the range of 0.05 to 0.5. However, there is a limit in wet-on-wetcoating adopted by the same patent to obtain the magnetic recordingmedium to suppress the interfacial fluctuation between a magnetic layerand a nonmagnetic layer, and when a magnetic layer is further thinned,the fluctuation at interface unfavorably causes noises. In addition, thelaminated Sendust head as used in the patent is an inductive head, butan MR head for use in higher density recording in the future is moresusceptible to the influence of a medium noise, hence the interfacialfluctuation will be actualized.

[0007] JP-A-2000-173038 ensures high electromagnetic characteristics andhigh durability by setting the thickness of a magnetic layer at 0.05 to0.5 μm and adding diamond particles having an average particle size offrom 0.05 to 1.0 μm to the magnetic layer in proportion of from 0.1 to 5mass % (i.e., weight %) based on the amount of the ferromagnetic metalpowder. However, the layer thickness in the Examples of the above patentis mainly restricted to 0.3 μm, and it is necessary to make magneticlayer thickness thinner for achieving higher density. Further, the layerthickness of about 0.3 μm is not liable to be influenced by theinterface between a magnetic layer and a nonmagnetic layer, and S/Nratio does not deteriorate even by wet-on-wet coating.

[0008] The evaluation of the magnetic recording media disclosed in thesepatents was performed by recording and reproducing using an MIG(metal-in-gap) head which is an inductive type magnetic head and trackdensity is relatively low, and when higher density recording is done byfurther lessening a track width or thinning the magnetic layerthickness, a sufficient S/N ratio cannot be obtained at reproduction. Inparticular, the influence of interfacial fluctuation becomes large whenan MR head is used, which causes the degradation of S/N ratio.

[0009] A low noise and high density magnetic recording medium and alsoexcellent in running durability is desired even when a conventionallyused coating-type magnetic recording medium which is excellent inproductivity and inexpensive is combined with an MR head.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide a magneticrecording medium which is excellent in electromagnetic characteristicsand is optimal for digital recording.

[0011] The present invention provides amagnetic recording mediumcomprising a support having provided thereon a substantially nonmagneticlower layer by coating a lower layer coating solution comprising anonmagnetic powder dispersed in a binder and drying, and a magneticlayer having a thickness of from 0.01 to 0.15 μm by coating a magneticlayer coating solution comprising a ferromagnetic metal powder or ahexagonal ferrite powder dispersed in a binder, wherein the magneticlayer contains diamond particles having an average particle size of from⅕ to 2 times the thickness of the magnetic layer in an amount of from0.1 to 5.0 mass % (i.e., weight %) based on the amount of theferromagnetic powder.

[0012] The preferred embodiments of the present invention are asfollows.

[0013] (1) The magnetic recording medium, wherein the lower layer has anaverage surface roughness of 20 nm or less.

[0014] (2) The magnetic recording medium which is for reproduction usingan MR head.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention relates to a magnetic recording mediumproduced by wet-on-dry coating of providing a magnetic layer on a lowerlayer after the lower layer has been dried.

[0016] The magnetic layer in the present invention has a thicknesspreferably of from 0.01 to 0.15 μm, more preferably from 0.03 to 0.10μm, a ferromagnetic metal powder or a hexagonal ferrite powder iscontained in the magnetic layer as a ferromagnetic powder, and diamondparticles having an average particle size of from ⅕ to 2 times thethickness of the magnetic layer are contained in the magnetic layer inan amount of from 0.1 to 5.0 mass % (i.e., weight %) based on the amountof the ferromagnetic powder.

[0017] The present invention can make the average surface roughness of alower layer preferably 20 nm or less, more preferably 10 nm or less, cansuppress the fluctuation at interface between a magnetic layer and anonmagnetic layer, can improve the S/N ratio by reproduction with an MRhead, and can ensure the running durability by specifying the particlesize of the diamond particles, prescribing the magnetic layer thicknessas thin as the above range, and adopting the wet-on-dry coating method.

[0018] The fluctuation at interface can be evaluated by electron raymicro-analysis (EPMA) as described in the Examples later.

[0019] In the present invention, when the magnetic layer containsdiamond particles having an average particle size of less than ⅕ timesthe thickness of the magnetic layer in an amount of less than 0.1 mass %based on the amount of the ferromagnetic powder, running durabilitycannot be ensured. When the magnetic layer contains diamond particleshaving an average particle size of less than ⅕ times the thickness ofthe magnetic layer in an amount of more than 5.0 mass % based on theamount of the ferromagnetic powder, S/N ratio cannot be improved. Whenthe magnetic layer contains diamond particles having an average particlesize of more than 2 times the thickness of the magnetic layer in anamount of less than 0.1 mass % based on the amount of the ferromagneticpowder, S/N ratio and running durability cannot be improved. When themagnetic layer contains diamond particles having an average particlesize of more than 2 times the thickness of the magnetic layer in anamount of more than 5.0 mass % based on the amount of the ferromagneticpowder, S/N ratio cannot be improved and running durability cannot beimproved furthermore.

[0020] Further, it is difficult to make the thickness of a magneticlayer less than 0.01 μm in good yield according to the presenttechniques by adding diamond particles having an average particle sizeof from ⅕ to 2 times the thickness of the magnetic layer to the magneticlayer in an amount of from 0.1 to 5.0 mass % based on the amount of theferromagnetic powder. Further, making the thickness of a magnetic layermore than 0.15 μm by adding diamond particles having an average particlesize of from ⅕ to 2 times the thickness of the magnetic layer to themagnetic layer in an amount of from 0.1 to 5.0 mass % based on theamount of the ferromagnetic powder cannot improve S/N ratio due to selfdemagnetization loss.

[0021] The constitutional elements of the present invention aredescribed in detail below.

[0022] Diamond Particles

[0023] Diamond particles for use in the present invention have anaverage particle size of from ⅕ to 2 times the thickness of the magneticlayer, preferably from ½ to 1.5 times, and more preferably from 0.8 to1.2 times. The amount of addition of diamond particles is from 0.1 to5.0 mass % based on the amount of the ferromagnetic powder, preferablyfrom 0.5 to 3 mass %. When the addition amount is less than 0.1 mass %,running durability cannot be ensured, while when it is more than 5.0mass %, S/N ratio lowers and head abrasion increases.

[0024] Diamond particles may be used in combination with aluminaparticles. The average particle size of alumina particles to be used incombination is from 0.05 to 0.5 μm, preferably from 0.1 to 0.3 μm. Theaddition amount of alumina particles is preferably from 1 to 30 mass %based on the amount of the ferromagnetic metal powders, more preferablyfrom 3 to 15 mass %. When the addition amount of alumina particles isless than 1 mass %, running durability cannot be ensured, while when theamount is more than 30 mass %, S/N ratio lowers and head abrasionincreases. In the present invention, diamond particles and aluminaparticles are added to the magnetic layer in a mass ratio ofdiamond/alumina of preferably from 1/0.5 to 1/30, more preferably from1/1.5 to 1/20. Even if the amount of alumina is too little or too largein the above addition rate, the improving effect of running durabilitylowers. The largest particle size of diamond particles or aluminaparticles is taken as the particle size in the present invention, andthe average value of the measured values of 500 particles sampled atrandom from electron microphotographs is taken as the average particlesize.

[0025] With respect to particle size distribution of diamond particles,it is preferred that the number of particles having the particle size of200% or more of the average particle size accounts for 5% or less of theentire number of diamond particles, and the number of particles havingthe particle size of 50% or less of the average particle size accountsfor 20% or less of the entire number of diamond particles. The maximumvalue of the particle size of the diamond particles for use in thepresent invention is about 0.8 μm, preferably about 0.4 μm, and theminimum value is about 0.005 μm, preferably about 0.01 μm.

[0026] Particle size distribution is obtained by counting the numbers ofrespective sizes based on the average particle size at the measurementof particle sizes. Particle size distribution of diamond particles alsoinfluences running durability, noise and head abrasion. If the particlesize distribution is broader than the above-described range, the effectsas described above deviate, i.e., when too large particle sizespredominate in the particle size distribution, head abrasion increasesin some cases. While when too small particle sizes predominate, theimproving effect of running durability sometimes lowers. Further,diamond particles having extremely narrow particle size distribution areexpensive, hence the above-described range is economically advantageousas well. The amount of abrasives can be largely decreased due to thecombined use with an alumina, which is not only advantageous from theviewpoint of the improvement of S/N ratio but also advantageous in thatthe design of a magnetic disc can be performed as desired, e.g., runningdurability is prior to S/N ratio and vice versa, making good use ofrelatively wide range of the addition amount of an alumina.

[0027] Magnetic Layer

[0028] The magnetic recording medium according to the present inventionmay be provided with a magnetic layer on either one side of a support ormay be provided on both sides.

[0029] The magnetic layer provided on one side of a support may be amonolayer or multilayers comprising different compositions from eachother. In the present invention, a substantially nonmagnetic lower layer(a nonmagnetic layer or a lower layer) is provided between the magneticlayer and a support by wet-on-dry coating. The magnetic layer is calledan upper layer or an upper magnetic layer.

[0030] Ferromagnetic Metal Powder

[0031] The ferromagnetic metal powders for use in the present inventionare preferably ferromagnetic metal powders mainly comprising α-Fe as amain component. These ferromagnetic metal powders may contain, inaddition to the prescribed atoms, the following atoms, e.g., Al, Si, Ca,Mg, Ti, Cr, Cu, Y, Sn, Sb, Ba, W, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Srand B. In particular, it is preferred to contain at least one of Al, Ca,Mg, Y, Ba, La, Nd, Sm, Co and Ni, in addition to α-Fe. The alloy of Cowith Fe is particularly preferred to increase saturation magnetizationand improve demagnetization. The content of Co is preferably from 1 to40 atomic %, more preferably from 15 to 35 atomic %, and most preferablyfrom 20 to 35 atomic %, the content of rare earth elements such as Y ispreferably from 1.5 to 12 atomic %, more preferably from 3 to 10 atomic%, and most preferably from 4 to 9 atomic %, and the content of Al ispreferably from 1.5 to 12 atomic %, more preferably from 3 to 10 atomic%, and most preferably from 4 to 9 atomic %, each based on Fe. Rareearth including Y and Al function as a sintering preventing agent, and ahigher sintering preventing effect can be obtained when they are used incombination. These ferromagnetic powders may be previously treated withthe later-described dispersants, lubricants, surfactants and antistaticagents before dispersion. Specifically, the examples are disclosed inJP-B-44-14090 (the term “JP-B” as used herein means an “examinedJapanese patent publication”), JP-B-45-18372, JP-B-47-22062,JP-B-47-22513, JP-B-46-28466, JP-B-46-38755, JP-B-47-4286,JP-B-47-12422, JP-B-47-17284, JP-B-47-18509, JP-B-47-18573,JP-B-39-10307, JP-B-46-39639, U.S. Pat. Nos. 3,026,215, 3,031,341,3,100,194, 3,242,005 and 3,389,014.

[0032] The ferromagnetic metal powders may contain a small amount of ahydroxide or an oxide. Ferromagnetic metal powders prepared bywell-known methods can be used, such as a method of reducingamoisture-containing iron oxide havingbeen subjected to sinteringinhibiting treatment or an iron oxide with a reducing gas, e.g.,hydrogen, to thereby obtain Fe or Fe—Co particles; a method of reducinga composite organic acid salt (mainly an oxalate) with a reducing gas,e.g., hydrogen; a method of heat-decomposing a metal carbonyl compound;a method of adding to an aqueous ferromagnetic metal solution a reducingagent, e. g., sodium boron hydride, hypophosphite or hydrazine, toeffect reduction; and a method of evaporating a metal in a low pressureinert gas to obtain a powder. The thus-obtained ferromagnetic metalpowders are subjected to well-known gradual oxidation treatment. Amethod of forming oxide films on the surfaces of ferromagnetic metalpowders by reducing a moisture-containing iron oxide or an iron oxidewith a reducing gas, e.g., hydrogen, and regulating partial pressures ofan oxygen-containing gas and an inert gas, temperature and time is lessin demagnetization and preferably used in the present invention.

[0033] The ferromagnetic metal powders for use in the magnetic layer inthe present invention have a specific surface area (S_(BET)) as measuredby the BET method of from 40 to 80 m²/g, preferably from 45 to 70 m²/g.When S_(BET) is less than 40 m²/g, noise increases, and when more than80 m²/g, a smooth surface is obtained with difficulty, which is notpreferred. The ferromagnetic metal powders for use in the magnetic layeraccording to the present invention have a crystallite size of generallyfrom 80 to 180 Å, preferably from 100 to 170 Å, and more preferably from110 to 165 Å. The average long axis length of the ferromagnetic metalpowders is generally from 0.02 to 0.25 μm, preferably from 0.03 to 0.15μm, and more preferably from 0.03 to 0.12 μm. The ferromagnetic metalpowders preferably have an average acicular ratio (the average of (longaxis length/short axis length)) of from 3 to 15, more preferably from 3to 10. The ferromagnetic metal powders have a saturation magnetization(σ_(s)) of generally from 90 to 170 A·m²/kg, preferably from 100 to 160A·m²/kg, and more preferably from 110 to 160 A·m²/kg. The ferromagneticmetal powders have a coercive force of preferably from 135 to 279 kA/m,and more preferably from 143 to 239 kA/m.

[0034] The ferromagnetic metal powders preferably have a moisturecontent of from 0.1 to 2 mass %. The moisture content of theferromagnetic metal powders is preferably optimized by selecting thekinds of binders. The pH of the ferromagnetic metal powders ispreferably optimized by the combination with the binder to be used. ThepH range is from 6 to 12, preferably from 7 to 11. The SA (stearic acid)adsorption amount of the ferromagnetic metal powders (the standard ofthe basic point of the surface) is from 1 to 15 μmol/m², preferably from2 to 10 μmol/m², and more preferably from 3 to 8 μmol/m². Whenferromagnetic metal powders high in a stearic acid adsorption amount areused, it is preferred to manufacture a magnetic recording medium bymodifying the surfaces of ferromagnetic metal powders with organicsubstances which are strongly adsorbed onto the surfaces thereof.Soluble inorganic ions (e.g., Na, Ca, Fe, Ni, Sr, NH₄, SO₄, Cl, NO₂ andNO₃) are sometimes contained in the ferromagnetic metal powders. It ispreferred that such soluble inorganic ions are substantially notcontained, but the properties of ferromagnetic metal powders are notparticularly affected if the total content of each ion is 300 ppm orless or so. The ferromagnetic metal powders for use in the presentinvention preferably have less voids and the value thereof is preferably20% by volume or less, more preferably 5% by volume or less. The shapeof the ferromagnetic metal powders is not particularly restricted, andany shape such as an acicular shape, an ellipsoidal shape and a spindleshape may be used so long as the above-described particle sizes andmagnetic characteristics are satisfied. Switching field distribution(SFD) of the ferromagnetic metal powders themselves is preferably small.It is necessary to make Hc distribution of the ferromagnetic metalpowders narrow. When the SFD of a tape is small, reversal ofmagnetization (i.e., magnetic flux revolution) becomes sharp and peakshift is small, which is, therefore, suitable for high density digitalmagnetic recording. For achieving small Hc distribution, making particlesize distribution of goethite in the ferromagnetic metal powders good,using monodispersed α-Fe₂O₃, and preventing sintering among particlesare effective methods.

[0035] Hexagonal Ferrite Powder

[0036] The examples of the hexagonal ferrite powders for use in themagnetic layer in the present invention include substitution products ofbarium ferrite, strontium ferrite, lead ferrite and calcium ferrite, andCo substitution products. Specifically, barium ferrite and strontiumferrite of magnetoplumbite type, magnetoplumbite type ferrite havingcovered the particle surfaces with spinel, and barium ferrite andstrontium ferrite of magnetoplumbite type partially containing a spinelphase are exemplified. The hexagonal ferrite powders may contain, inaddition to the prescribed atoms, the following atoms, e.g., Al, Si, S,Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg,Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge and Nb. In general,the hexagonal ferrite powders containing the following elements can beused, e.g., Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co,Sb—Zn—Co, and Nb—Zn. According to starting materials and producingprocesses, specific impurities may be contained.

[0037] The average tabular diameter of the hexagonal ferrite powders foruse in the present invention is generally from 10 to 50 nm, preferablyfrom 10 to 40 nm, and particularly preferably from 10 to 35 nm, althoughit varies according to recording density. The tabular diameter used heremeans the longest hexagonal diameter of the base of a hexagonal pole ofa hexagonal ferrite magnetic powder, and the average tabular diameter isthe arithmetic mean of it.

[0038] When reproduction is performed using a magneto-resistance headparticularly for increasing track density, it is necessary to reducenoise, accordingly the tabular diameter is preferably 35 nm or less, butwhen the tabular diameter is smaller than 10 nm, stable magnetizationcannot be obtained due to thermal fluctuation. While when the tabulardiameter exceeds 50 nm, noise increases, therefore, none of such tabulardiameters are suitable for high density recording. A tabular ratio(tabular diameter/tabular thickness) is preferably from 1 to 15, morepreferably from 1 to 7. When a tabular ratio is small, the packingdensity in a magnetic layer becomes high, which is preferred butsufficient orientation cannot be obtained. When a tabular ratio is morethan 15, noise increases due to stacking among particles. The specificsurface area (S_(BET)) measured by the BET method of the particleshaving diameters within this range is generally from 30 to 200 m²/g. Thespecific surface areas nearly coincide with the values obtained byarithmetic operations from tabular diameters and tabular thicknesses.The distribution of tabular diameter/tabular thickness is in generalpreferably as narrow as possible. The distributions in numerical valuescan be compared by measuring TEM photographs of 500 particles selectedrandomly.

[0039] The distribution is in many cases not regular distribution, butwhen expressed by the standard deviation to the average diameter bycomputation, σ/average diameter is from 0.1 to 2.0. For obtaining narrowparticle size distribution, it is efficient to make a particle-formingreaction system homogeneous to the utmost, particles formed aresubjected to distribution-improving treatments as well. For example, amethod of selectively dissolving ultrafine particles in an acid solutionis also known. Coercive force (Hc) measured in magnetic powders of about500 to about 5,000 Oe (=about 40 to 400 kA/m) can be produced. Higher Hcis advantageous for high density recording but it is restricted by thecapacities of recording heads. Hc can be controlled by particlediameters (tabular diameter and tabular thickness), the kinds andamounts of elements contained, the substitution sites of elements, andthe reaction conditions of particle formation. Saturation magnetization(σ_(s)) is from 30 to 80 A·m²/kg. σ_(s) has the inclination of becomingsmaller as particles become finer. A method of reducing thecrystallization temperature or heat treatment temperature and time, amethod of increasing the amount of the compound to be added, or a methodof increasing the surface treating amount can be used in manufacturing.A W-type hexagonal ferrite can also be used. Further, when magneticpowders are dispersed, the particle surfaces of the magnetic powders maybe treated with substances compatible with the dispersion media and thepolymers.

[0040] Inorganic or organic compounds are used as a surface treatingagent. Oxides or hydroxides of Si, Al and P, various kinds of silanecoupling agents, and various kinds of titanium coupling agents are therepresentatives of such a surface treating agent, for example. Theamount of the surface treating agent is from 0.1 to 10% based on theamount of the magnetic powder. The pH of magnetic powders is alsoimportant for dispersion, and is in general from 4 to 12. The optimalvalue is dependent upon the dispersion medium and the polymer. Takingthe chemical stability and the storage stability of the medium intoconsideration, pH of from about 6 to about 11 or so is selected. Thewater content in the magnetic powder also affects dispersion. Theoptimal value of the water content is dependent upon the dispersionmedium and the polymer, and is generally from 0.01 to 2.0%. Hexagonalferrite powders for use in the present invention are produced by any of(1) a glass crystallization method of mixing a metallic oxide whichsubstitutes barium oxide, iron oxide and iron, and a boron oxide as aglass-forming material, so as to make a desired ferrite composition,melting and quenching to obtain an amorphous product, reheat-treatingthe obtained product, washing and then pulverizing, to thereby obtain abarium ferrite crystal powder, (2) a hydrothermal reaction method ofneutralizing a solution of barium ferrite composition metal salt with analkali, removing the byproducts, heating the liquid phase at 100° C. ormore, washing, drying and pulverizing, to thereby obtain a bariumferrite crystal powder, and (3) a coprecipitation method of neutralizinga solution of barium ferrite composition metal salt with an alkali,removing the byproducts, drying, treating at 1,100° C. or less, and thenpulverizing, to thereby obtain a barium ferrite crystal powder, and anyof these methods can be used in the present invention.

[0041] Lower Layer

[0042] The lower layer is described in detail below. The lower layerpreferably comprises a nonmagnetic inorganic powder and a binder as maincomponents. The nonmagnetic inorganic powder for use in the lower layercan be selected from inorganic compounds, e.g., metallic oxide, metalliccarbonate, metallic sulfate, metallic nitride, metallic carbide andmetallic sulfide. The examples of inorganic compounds are selected fromthe following compounds and they can be used alone or in combination,e.g., α-alumina having an alpha-conversion rate of 90% or more,β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, ceriumoxide, α-ironoxide, hematite, goethite, corundum, siliconnitride,titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesiumoxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide,calcium carbonate, calcium sulfate, barium sulfate, and molybdenumdisulfide. Of these compounds, titanium dioxide, zinc oxide, iron oxideand barium sulfate are particularly preferred because they have smallparticle size distribution and various means for imparting functions,and titanium dioxide and α-iron oxide are more preferred. Thesenonmagnetic inorganic powders preferably have an average particle sizeof from 0.005 to 2 μm. If necessary, a plurality of nonmagneticinorganic powders each having a different particle size may be combined,or a single nonmagnetic inorganic powder may have broad particle sizedistribution so as to attain the same effect as such a combination.These nonmagnetic inorganic powders particularly preferably have anaverage particle size of from 0.01 to 0.2 μm. In particular, when thenonmagnetic inorganic powder is a granular metallic oxide, the averageparticle size thereof is preferably 0.08 μm or less, and when thenonmagnetic inorganic powder is an acicular metallic oxide, the averagelong axis length thereof is preferably 0.3 μm or less, more preferably0.2 μm or less. The Nonmagnetic inorganic powders for use in the presentinvention have a tap density of generally from 0.05 to 2 g/ml,preferably from 0.2 to 1.5 g/ml; a water content of generally from 0.1to 5 mass %, preferably from 0.2 to 3 mass %, and more preferably from0.3 to 1.5 mass %; a pH value of generally from 2 to 11, andparticularly preferably between 5.5 and 10; a specific surface area(S_(BET)) of generally from 1 to 100 m²/g, preferably from 5 to 80 m²/g,and more preferably from 10 to 70 m²/g.

[0043] The Nonmagnetic inorganic powders for use in the presentinvention have a crystallite size of preferably from 0.004 to 1 μm, andmore preferably from 0.04 to 0.1 μm; an oil absorption amount using DBP(dibutyl phthalate) of generally from 5 to 100 ml/100 g, preferably from10 to 80 ml/100 g, and more preferably from 20 to 60 ml/100 g; and aspecific gravity of generally from 1 to 12, and preferably from 3 to 6.The shape of the nonmagnetic inorganic powders may be any of anacicular, spherical, polyhedral, or tabular shape. The nonmagneticinorganic powders preferably have a Mohs' hardness of from 4 to 10. TheSA (stearic acid) adsorption amount of the nonmagnetic inorganic powdersis from 1 to 20 μmol/m², preferably from 2 to 15 μmol/m², and morepreferably from 3 to 8 μmol/m². The pH of the nonmagnetic inorganicpowders is preferably between 3 and 6. The surfaces of these nonmagneticinorganic powders can be covered with Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂,Sb₂O₃, ZnO or Y₂O₃. Al₂O₃, SiO₂, TiO₂ and ZrO₂ are particularlypreferred in the point of dispersibility, and Al₂O₃, SiO₂ and ZrO₂ aremore preferred. They can be used in combination or may be used alone.According to purpose, a layer subjected to surface treatment bycoprecipitation may be used. Alternatively, surface treatment ofparticles may be previously performed to be covered with alumina in thefirst place, then the alumina-covered surface is covered with silica, orvice versa, according to purposes. The surface-covered layer may beporous layer, if necessary, but a homogeneous and dense surface isgenerally preferred.

[0044] The specific examples of the nonmagnetic inorganic powders foruse in the lower layer in the present invention and the producingmethods are disclosed in WO 98/35345.

[0045] By adding carbon blacks to the lower layer, a desired microVickers' hardness can be obtained, surface electrical resistance (Rs)and light transmittance can be reduced as well, as are well-knowneffects. It is also possible to bring about the effect of stocking alubricant by adding carbon blacks to the lower layer. Furnace blacks forrubbers, thermal blacks for rubbers, carbon blacks for coloring andacetylene blacks can be used as carbon blacks. Carbon blacks for use inthe lower layer should optimize the following characteristics by thedesired effects and further effects can be obtained by the combined usein some cases.

[0046] Carbon blacks for use in the lower layer according to the presentinvention have a specific surface area (S_(BET)) Of generally from 100to 500 m²/g, preferably from 150 to 400 m²/g, a DBP oil absorptionamount of generally from 20 to 400 ml/100 g, preferably from 30 to 400ml/100 g, an average particle size of generally from 5 to 80 nm,preferably from 10 to 50 nm, and more preferably from 10 to 40 nm, and asmall amount of carbon blacks having an average particle size of largerthan 80 nm may be contained in the lower layer. Carbon blacks for use inthe lower layer have pH of from 2 to 10, a water content of from 0.1 to10%, and a tap density of from 0.1 to 1 g/ml.

[0047] The specific examples of the carbon blacks for use in the lowerlayer in the present invention are disclosed in WO 98/35345. Carbonblacks can be used within the range not exceeding 50 mass % of the abovenonmagnetic inorganic powders (exclusive of carbon blacks) and notexceeding 40% of the total mass of the nonmagnetic layer. These carbonblacks can be used alone or in combination. With respect to the carbonblacks which can be used in the present invention, e.g., the disclosurein Carbon Black Binran (Handbook of Carbon Blacks), compiled by CarbonBlack Kyokai can be referred to.

[0048] Organic powders can be used in the lower layer according topurposes. The examples of organic powders include an acryl styrene resinpowder, a benzoguanamine resin powder, a melamine resin powder, and aphthalocyanine pigment. In addition, a polyolefin resin powder, apolyester resin powder, a polyamide resin powder, apolyimide resinpowder, and a polyethylene fluoride resin powder can also be used. Theproducing methods of these organic powders are disclosed inJP-A-62-18564 and JP-A-60-255827.

[0049] The binder resins, lubricants, dispersants, additives, solvents,dispersing methods, and others used for the magnetic layer describedbelow can be used in the lower layer and the backing layer describedlater. In particular, with respect to the amounts and the kinds ofbinder resins, additives, the amounts and the kinds of dispersants,well-known prior art techniques regarding the magnetic layer can beapplied to the lower layer.

[0050] Binder

[0051] Conventionally well-known thermoplastic resins, thermosettingresins, reactive resins and mixtures of these resins are used as abinder in the present invention.

[0052] Thermoplastic resins having a glass transition temperature offrom −100 to 150° C., a number average molecular weight of from 1,000 to200,000, preferably from 10,000 to 100,000, and a polymerization degreeof from about 50 to about 1,000 can be used in the present invention.

[0053] The examples of thermoplastic resins include polymers orcopolymers containing, as the constituting unit, vinyl chloride, vinylacetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid ester,vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic acidester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal orvinyl ether; polyurethane resins and various rubber resins. The examplesof thermosetting resins and reactive resins include phenolic resins,epoxy resins, curable type polyurethane resins, urea resins, melamineresins, alkyd resins, acrylic reactive resins, formaldehyde resins,silicone resins, epoxy-polyamide resins, mixtures of polyester resinsand isocyanate prepolymers, mixtures of polyesterpolyol andpolyisocyanate, and mixtures of polyurethane and polyisocyanate. Theseresins are described in detail in Plastic Handbook, Asakura Shoten. Itis also possible to use well-known electron beam-curable type resins ineach layer. The examples of these resins and producing methods aredisclosed in detail in JP-A-62-256219. These resins can be used alone ormay be used in combination. The examples of preferred combinationsinclude at least one resin selected from vinyl chloride resins, vinylchloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinylalcohol copolymers, and vinyl chloride-vinyl acetate-maleic anhydridecopolymers with polyurethane resins, and combinations of these resinswith polyisocyanate.

[0054] As the polyurethane resins, those having well-known structures,e.g., polyester polyurethane, polyether polyurethane, polyetherpolyester polyurethane, polycarbonate polyurethane, polyesterpolycarbonate polyurethane, and polycaprolactone polyurethane can beused. For the purpose of further improving the dispersibility and thedurability, it is referred that at least one polar group selected fromthe following is introduced by copolymerization or addition reaction,with respect to all the binders described above, e.g., —COOM, —SO₃M,—OSO₃M, —P═O(OM)₂, —O—P═O(OM)₂ (wherein M represents a hydrogen atom oran alkali metal salt group), —NR₂, —N⁺R₃ (R represents a hydrocarbongroup), an epoxy group, —SH, and —CN. The content of the polar group isfrom 10⁻¹ to 10⁻⁸ mol/g, preferably from 10⁻² to 10⁻⁶ mol/g. It ispreferred that polyurethane resins have at least one OH group at eachterminal of a polyurethane molecule, i.e., two or more in total, besidesthe above polar groups. As OH groups form three dimensional networkstructure by crosslinking with a polyisocyanate curing agent, they arepreferably contained in a molecule as many as possible. In particular,it is preferred that OH groups are present at terminals of a molecule,since the reactivity with the curing agent becomes high. It is preferredfor polyurethane to have 3 or more OH groups, particularly preferably 4or more OH groups, at terminals of a molecule. When polyurethane is usedin the present invention, the polyurethane has a glass transitiontemperature of generally from −50 to 150° C., preferably from 0 to 100°C., and particularly preferably from 30 to 100° C., breaking extensionof from 100 to 2,000%, breaking stress of generally from 0.05 to 10kg/mm² (=about 0.49 to 98 MPa), and a yielding point of from 0.05 to 10kg/mm² (=about 0.49 to 98 MPa). Due to these physical properties, acoated film having good mechanical properties can be obtained.

[0055] The specific examples of the binders for use in the presentinvention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC,XYHL, XYSG, PKHH, PKHJ, PKHC and PKFE (manufactured by Union CarbideCo., Ltd.), MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TMand MPR-TAO (manufactured by Nisshin Chemical Industry Co., Ltd.),1000W, DX80, DX81, DX82, DX83 and 100FD (manufactured by ElectroChemical Industry Co., Ltd.), MR-104, MR-105, MR-110, MR-100, MR-555 and400X-110A (manufactured by Nippon Zeon Co., Ltd.) as vinyl chloridecopolymers; Nippollan N2301, N2302 and N2304 (manufactured by NipponPolyurethane Co., Ltd.), Pandex T-5105, T-R3080, T-5201, Burnock D-400,D-210-80, Crisvon 6109 and 7209 (manufactured by Dainippon Chemicals andInk. Co., Ltd.), Vylon UR8200, UR8300, UR8700, RV530 and RV280(manufactured by Toyobo Co., Ltd.), polycarbonate polyurethane,Daipheramine 4020, 5020, 5100, 5300, 9020, 9022 and 7020 (manufacturedby Dainichi Seika K.K.), polyurethane, MX5004 (manufactured byMitsubishi Kasei Corp.), polyurethane, Sunprene SP-150 (manufactured bySanyo Chemical Industries Co. Ltd.), polyurethane, Salan F310 and F210(manufactured by Asahi Chemical Industry Co., Ltd.) as polyurethaneresins, etc.

[0056] The amount of the binder for use in the nonmagnetic layer and themagnetic layer in the present invention is from 5 to 50 mass % (i.e.,weight %), preferably from 10 to 30 mass %, based on the weight of thenonmagnetic inorganic powder and the magnetic powder respectively. Whenvinyl chloride resins are used, the amount thereof is from 5 to 30 mass%, when polyurethane resins are used, the amount thereof is from 2 to 20mass %, and it is preferred that polyisocyanate is used in an amount offrom 2 to 20 mass % in combination with these resins, however, forinstance, when the corrosion of a head is caused by a slight amount ofchlorine due to dechlorination, it is possible to use polyurethane aloneor a combination of polyurethane and isocyanate alone.

[0057] The magnetic recording medium in the present invention comprisetwo or more layers. Accordingly, the amount of the binder, the amountsof the vinyl chloride resin, polyurethane resin, polyisocyanate or otherresins contained in the binder, the molecular weight of each resinconstituting the magnetic layer, the amount of polar groups, or theabove-described physical properties of resins can of course be varied inthe nonmagnetic layer and the magnetic layer according to necessity.These factors should be rather optimized in respective layers.Well-known techniques with respect to multilayer magnetic layers can beused in the present invention. For example, when the amount of thebinder is varied in each layer, it is effective to increase the amountof the binder contained in the magnetic layer to reduce scratches on thesurface of the magnetic layer. For improving the head touch against thehead, it is effective to increase the amount of the binder in thenonmagnetic layer to impart flexibility.

[0058] The examples of the polyisocyanates for use in the presentinvention include isocyanates, e.g., tolylenediisocyanate,4,4′-diphenylmethanediisocyanate, hexamethylenediisocyanate,xylylenediisocyanate, naphthylene-1,5-diisocyanate,o-toluidinediisocyanate, isophoronediisocyanate, andtriphenylmethanetriisocyanate; reaction products of these isocyanateswith polyalcohols; and polyisocyanates formed by condensation reactionof isocyanates. These polyisocyanates are commercially available underthe trade names of Coronate L, Coronate HL, Coronate 2030, Coronate2031, Millionate MR and Millionate MTL (manufactured by NipponPolyurethane Co., Ltd.), Takenate D-102, Takenate D-110N, Takenate D-200and Takenate D-202 (manufactured by Takeda Chemical Industries, Ltd.),and Desmodur L, Desmodur IL, Desmodur N and Desmodur HL (manufactured bySumitomo Bayer Co., Ltd.). These polyisocyanates may be used alone or incombinations of two or more thereof in each layer, taking advantage of adifference in curing reactivity.

[0059] Carbon Black, Abrasive

[0060] The carbon blacks for use in the magnetic layer according to thepresent invention include furnace blacks for rubbers, thermal blacks forrubbers, carbon blacks for coloring, and acetylene blacks. Carbon blacksfor use in the magnetic layer of the present invention preferably have aspecific surface area (S_(BET)) of from 5 to 500 m²/g, a DBP oilabsorption amount of from 10 to 400 ml/100 g, an average particle sizeof from 5 to 300 nm, pH of from 2 to 10, a water content of from 0.1 to10%, and a tap density of from 0.1 to 1 g/ml. The specific examples ofthe carbon blacks for use in the magnetic layer of the present inventionare disclosed in WO 98/35345.

[0061] Carbon blacks can serve various functions such as prevention ofstatic charges of the magnetic layer, reduction of a frictioncoefficient, impartation of a light-shielding property and improvementof film strength. Such functions vary depending upon the kind of carbonblacks to be used. Accordingly, when the present invention takes amultilayer structure, it is of course possible to select and determinethe kinds, the amounts and the combinations of the carbon blacks to beadded to each layer on the basis of the above-described variousproperties such as the particle size, the oil absorption amount, theelectroconductivity and the pH value, or these should be ratheroptimized in respective layers.

[0062] Abrasives other than diamond particles can be used in combinationin the magnetic layer and other layers according to the presentinvention. Well-known materials essentially having a Mohs' hardness of 6or more are used alone or in combination as the abrasive in the magneticlayer according to the present invention. The examples of such abrasivesinclude α-alumina having an alpha-conversion rate of 90% or more,β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide,corundum, silicon nitride, silicon carbide, titanium carbide, titaniumoxide, silicon dioxide, and boron nitride. The composites composed ofthese abrasives (abrasives obtained by surface-treating with otherabrasives) may also be used. Compounds or elements other than the maincomponent are often contained in the abrasives, but the intended effectcan be attained so far as the content of the main component is 90% ormore. The abrasives preferably have an average particle size of from0.0to 2 μm and, in particular, for improving electromagneticcharacteristics, it is preferred to use abrasives having narrow particlesize distribution. For improving durability, abrasives each having adifferent particle size may be combined according to necessity, or asingle abrasive having a broad particle size distribution may be used soas to attain the same effect as such a combination. Preferably, theabrasives for use in the present invention have a tap density of from0.3 to 2 g/ml, a water content of from 0.1 to 5%, a pH value of from 2to 11, and a specific surface area (S_(BET)) of from 1 to 30 m²/g. Theshape of the abrasives for use in the present invention may be any ofacicular, spherical and die-like shapes. Abrasives having a shape partlywith edges are referred, because a high abrasive property can beobtained. The specific examples of the abrasives for use in the magneticlayer of the present invention are disclosed in WO 98/35345. Theparticle size and the amount of the abrasive to be added to the magneticlayer and the nonmagnetic layer should be selected at optimal valuesindependently.

[0063] Additive

[0064] As the additives for use in the magnetic layer and thenonmagnetic layer of the present invention, those having a lubricatingeffect, an antistatic effect, a dispersing effect and a plasticizingeffect are used, and comprehensive improvement of capacities can becontrived by combining the additives. As the additives having alubricating effect, lubricants remarkably act on cohesion caused by thefriction of surfaces of materials with each other are used. Lubricantsare classified into two types. Lubricants which are used for a magneticrecording medium cannot be judged completely whether they show fluidlubrication or boundary lubrication, but according to general conceptsthey are classified into higher fatty acid esters, liquid paraffins andsilicon derivatives which show fluid lubrication, and long chain fattyacids, fluorine surfactants and fluorine-containing high polymers whichshow boundary lubrication. In a coating type magnetic recording medium(i.e., a magnetic recording particulate medium), lubricants exist in astate dissolved in a binder or in a state partly adsorbed onto thesurface of a ferromagnetic powder, and they migrate to the surface of amagnetic layer. The speed of migration depends upon whether thecompatibility of a binder and a lubricant is good or bad. The speed ofmigration is slow when the compatibility of a binder and a lubricant isgood and the migration speed is fast when the compatibility is bad. Asone idea as to good or bad of the compatibility, there is a means ofcomparison of dissolution parameters of a binder and a lubricant. Anonpolar lubricant is effective for fluid lubrication and a polarlubricant is effective for boundary lubrication.

[0065] In the present invention, it is preferred to use a higher fattyacid ester showing fluid lubrication and a long chain fatty acid showingboundary lubrication each having different characteristics incombination, and it is more preferred to combine at least three of theselubricants. Solid lubricants can also be used in combination with theselubricants.

[0066] The examples of the solid lubricants which can be used incombination include molybdenum disulfide, tungsten graphite disulfide,boron nitride, and graphite fluoride. The examples of the long chainfatty acids showing boundary lubrication include monobasic fatty acidshaving from 10 to 24 carbon atoms (which may contain an unsaturated bondor may be branched) and metal salts of these monobasic fatty acids(e.g., with Li, Na, K or Cu). The examples of the fluorine surfactantsand fluorine-containing high polymers include fluorine-containingsilicones, fluorine-containing alcohols, fluorine-containing esters,fluorine-containing alkyl sulfates and alkali metal salts of them. Theexamples of the higher fatty acid esters showing fluid lubricationinclude basic fatty acid monoesters, fatty acid diesters or fatty acidtriesters composed of a monobasic fatty acid having from 10 to 24 carbonatoms (which may contain an unsaturated bond or may be branched) and anyone of mono-, di-, tri-, tetra-, penta- and hexa-alcohols having from 2to 12 carbon atoms (which may contain an unsaturated bond or may bebranched), and fatty acid esters of monoalkyl ethers of alkylene oxidepolymers. In addition to the above, the examples further include liquidparaffins, and as the silicon derivatives, silicone oils, e.g.,dialkylpolysiloxane (the alkyl group has from 1 to 5 carbon atoms),dialkoxypolysiloxane (the alkoxy group has from 1 to 4 carbon atoms),monoalkylmonoalkoxypolysiloxane (the alkyl group has from 1 to 5 carbonatoms and the alkoxy group has from 1 to 4 carbon atoms),phenylpolysiloxane, and fluoroalkylpolysiloxane (the alkyl group hasfrom 1 to 5 carbon atoms), silicones having a polar group, fattyacid-modified silicones, and fluorine-containing silicones.

[0067] The examples of other lubricants which can be used in the presentinvention include alcohols such as mono-, di-, tri-, tetra-, penta- orhexa-alcohols having from 12 to 22 carbon atoms (which may contain anunsaturated bond or may be branched), alkoxy alcohols having from 12 to22 carbon atoms (which may contain an unsaturated bond or may bebranched), and fluorine-containing alcohols, polyethylene waxes,polyolefins such as polypropylenes, ethylene glycols, polyglycols suchas polyethylene oxide waxes, alkyl phosphates and alkali metal salts ofalkyl phosphates, alkyl sulfates and alkali metal salts of alkylsulfates, polyphenyl ethers, fatty acid amides having from 8 to 22carbon atoms, and aliphatic amines having from 8 to 22 carbon atoms.

[0068] The examples of the additives having an antistatic effect, adispersing effect and a plasticizing effect which can be used in thepresent invention include phenylphosphonic acids, specifically “PPA”(manufactured by Nissan Chemical Industries, Ltd.), etc.,α-naphthylphosphoric acids, phenylphosphoric acids, diphenylphosphoricacids, p-ethylbenzenephosphonic acids, phenylphosphinic acids,aminoquinones, various kinds of silane coupling agents, titaniumcoupling agents, fluorine-containing alkyl sulfates and alkali metalsalts thereof.

[0069] The lubricants particularly preferably used in the presentinvention are fatty acids and fatty acid esters, and the specific aredisclosed in WO 98/35345. Besides the above, other different lubricantsand additives can be used in combination as well.

[0070] Additionally, nonionic surfactants, e.g., alkylene oxides,glycerols, glycidols and alkylphenol-ethylene oxide adducts; cationicsurfactants, e. g. , cyclic amines, ester amides, quaternary ammoniumsalts, hydantoin derivatives, heterocyclic compounds, phosphoniums andsulfoniums; anionic surfactants containing an acidic group, e.g.,carboxylic acid, sulfonic acid, phosphoric acid, sulfuric acid estergroups or phosphoric acid ester groups; and amphoteric surfactants,e.g., amino acids, aminosulfonic acids, sulfates or phosphates of aminoalcohols, and alkylbetains can also be used. These surfactants aredescribed in detail in Kaimen Kasseizai Binran (Handbook of Surfactants)(published by Sangyo Tosho Co., Ltd.). These lubricants and antistaticagents need not be 100% pure and may contain impurities such as isomers,non-reacted products, byproducts, decomposed products and oxides, inaddition to the main component. However, the content of such impuritiesis preferably 30% or less, more preferably 10% or less.

[0071] As described in WO 98/35345, it is also preferred to use amonoester and a diester in combination as the fatty acid ester.

[0072] The surface of the magnetic layer of the magnetic recordingmedium in the present invention, in particular, the disc-like magneticrecording medium, has a C/Fe peak ratio of preferably from 5 to 100,particularly preferably from 5 to 80, when measured by the Augerelectron spectroscopy. The measuring condition of the C/Fe peak ratio bythe Auger electron spectroscopy is as follows.

[0073] Apparatus: Model PHI-660 manufactured by φ Co.

[0074] Measuring condition:

[0075] Primary electron beam accelerating voltage: 3 KV

[0076] Electric current of sample: 130 nA

[0077] Magnification: 250-fold

[0078] Inclination angle: 30°

[0079] The value of C/Fe peak is obtained as the C/Fe ratio byintegrating the values obtained under the above conditions in the regionof kinetic energy of 130 eV to 730 eV three times and finding thestrengths of KLL peak of the carbon and LMM peak of the iron asdifferentials.

[0080] The amount of the lubricants contained in each of the upper layerand the lower layer of the magnetic recording medium according to thepresent invention is preferably from 5 to 30 mass parts per 100 massparts of the ferromagnetic powder or the nonmagnetic inorganic powder.

[0081] The lubricants and surfactants for use in the present inventionrespectively have different physical functions. The kinds, amounts andproportions of generating synergistic effect of these lubricants shouldbe determined optimally in accordance with the purpose. The nonmagneticlayer and the magnetic layer can separately contain different fattyacids each having a different melting point so as to prevent bleedingout of the fatty acids to the surface, or different esters each having adifferent boiling point, a different melting point or a differentpolarity so as to prevent bleeding out of the esters to the surface.Also, the amount of the surfactant is controlled so as to improve thecoating stability, or the amount of the lubricant in the intermediatelayer is made larger so as to improve the lubricating effect. Theexamples are by no means limited there to. In general, the total amountof the lubricants is from 0.1 to 50 mass %, preferably from 2 to 25 mass%, based on the amount of the magnetic powder or the nonmagneticinorganic powder.

[0082] All or a part of the additives to be used in the presentinvention may be added to the magnetic coating solution or thenonmagnetic coating solution in any step of the preparation. Forexample, the additives may be blended with the magnetic powder beforethe kneading step, may be added in the step of kneading the magneticpowder, the binder and the solvent, may be added in the dispersing step,may be added after the dispersing step, or may be added just beforecoating. According to the purpose, there is the case of capable ofattaining the object by coating all or a part of the additivessimultaneously with or successively after the coating of the magneticlayer. According to the purpose, the lubricants may be coated on thesurface of the magnetic layer after calendering treatment or aftercompletion of slitting.

[0083] Layer Structure

[0084] The thickness of the support of the magnetic recording medium inthe present invention is generally from 2 to 100 μm, preferably from 2to 80 μm. The thickness of the support of a computer tape is from 3.0 to6.5 μm, preferably from 3.0 to 6.0 μm, and more preferably from 4.0 to5.5 μm.

[0085] An undercoating layer may be provided between the support,preferably the nonmagnetic flexible support, and the nonmagnetic ormagnetic layer for adhesion improvement. The thickness of theundercoating layer is from 0.01 to 0.5 μm, preferably from 0.02 to 0.5μm.

[0086] A backing layer may be provided on the side of the supportopposite to the side having the magnetic layer for the purpose ofproviding static charge prevention and curling correction. The thicknessof the backing layer is generally from 0.1 to 4 μm, preferably from 0.3to 2.0 μm. Well-known undercoating layers and backing layers can be usedfor this purpose.

[0087] The thickness of the magnetic layer of the constitutioncomprising the lower layer and the upper layer in the present inventionis as described above and is optimized according to the saturationmagnetization amount of the head to be used, the head gap length, andthe recording signal zone. The thickness of the lower layer is generallyfrom 0.2 to 5.0 μm, preferably from 0.3 to 3.0 μm, and more preferablyfrom 1.0 to 2.5 μm.

[0088] The lower layer exhibits the effect of the present invention solong as it is substantially a nonmagnetic layer even if, or intendedly,it contains a small amount of a magnetic powder as an impurity, which isas a matter of course regarded as essentially the same constitution asin the present invention. The term “substantially a nonmagnetic layer”means that the residual magnetic flux density of the lower layer is 10mT or less or the coercive force of the lower layer is 100 Oe (=about 8kA/m) or less, preferably the residual magnetic flux density and thecoercive force are zero. When the lower layer contains a magneticpowder, the content of the magnetic layer is preferably less than ½ ofthe entire inorganic powder contained in the lower layer. As the lowerlayer, a soft magnetic layer containing a soft magnetic powder and abinder may be formed in place of the nonmagnetic layer. The thickness ofthe soft magnetic layer is the same as the thickness of the lower layerdescribed above.

[0089] Backing Layer

[0090] The magnetic recording medium in the present invention can beprovided with a backing layer. A magnetic disc may also be provided witha backing layer, however, in general, a magnetic tape for a computerdata recording is decidedly required to have an excellentrepeating-running property as compared with a video tape and an audiotape. It is preferred for the backing layer to contain a carbon blackand an inorganic powder for maintaining such a high running durability.

[0091] It is preferred to use two kinds of carbon blacks each havingdifferent average particle size in combination. In such a case, a finecarbon black having an average particle size of from 10 to 20 nm and acoarse carbon black having an average particle size of from 230 to 300nm are preferably used in combination. In general, by the addition of afine carbon black as above, the surface electrical resistance of thebacking layer can be set at a low value and light transmittance can alsobe set at a low value. Since there are many kinds of magnetic recordingapparatus making use of light transmittance of a tape to make it as asignal of operation, addition of fine carbon blacks is particularlyeffective in such a case. Further, fine carbon blacks are in generalexcellent in retention of a liquid lubricant and contribute to thereduction of a friction coefficient when a lubricant is used incombination. On the other hand, coarse carbon blacks having an averageparticle size of from 230 to 300 nm have a function as a solid lubricantand form minute protrusions on the surface of the backing layer toreduce the contact area and contribute to the reduction of a frictioncoefficient.

[0092] When commercially available products are used as the fine carbonblacks and coarse carbon blacks which can be used in the presentinvention, those disclosed in WO 98/35345 can be exemplified as thespecific examples.

[0093] When two kinds of carbon blacks each having different averageparticle size are used in combination in the backing layer, theproportion of the contents (by mass (i.e., by weight)) of a fine carbonblack having a particle size of from 10 to 20 nm and a coarse carbonblack having a particle size of from 230 to 300 nm is preferably theformer/the latter of from 98/2 to 75/25, more preferably from 95/5 to85/15.

[0094] The content of the carbon black in the backing layer (the totalamount when two kinds of carbon blacks are used) is generally from 30 to80 mass parts (i.e., weight parts), preferably from 45 to 65 mass parts,per 100 mass parts of the binder.

[0095] It is preferred to use two kinds of inorganic powders each havingdifferent hardness.

[0096] Specifically, a soft inorganic powder having a Mohs' hardness offrom 3 to 4.5 and a hard inorganic powder having a Mohs' hardness offrom 5 to 9 are preferably used in combination.

[0097] By the addition of a soft inorganic powder having a Mohs'hardness of from 3 to 4.5, a friction coefficient can be stabilizedagainst repeating-running. Moreover, a sliding guide pole is notscratched off with the hardness within this range. The average particlesize of such a soft inorganic powder is preferably from 30 to 50 nm.

[0098] The examples of soft inorganic powders having a Mohs' hardness offrom 3 to 4.5 include, e.g., calcium sulfate, calcium carbonate, calciumsilicate, barium sulfate, magnesium carbonate, zinc carbonate and zincoxide. These soft inorganic powders can be used alone or in combinationof two or more.

[0099] The content of the soft inorganic powder in a backing layer ispreferably from 10 to 140 mass parts, more preferably from 35 to 100mass parts, per 100 mass parts of the carbon black.

[0100] By the addition of a hard inorganic powder having a Mohs'hardness of from 5 to 9, the strength of a backing layer is increasedand running durability is improved. When such hard inorganic powders areused together with carbon blacks and the above-described soft inorganicpowders, deterioration due to repeating sliding is reduced and strongbacking layer can be obtained. Appropriate abrasive property is given tothe backing layer by the addition of the hard inorganic powder and theadhesion of scratched powders to a tape guide pole is reduced. Inparticular, when the hard inorganic powder is used in combination with asoft inorganic powder, sliding characteristics against a guide polehaving a rough surface is improved and the stabilization of a frictioncoefficient of the backing layer can also be brought about.

[0101] The average particle size of hard inorganic powders is preferablyfrom 80 to 250 nm, more preferably from 100 to 210 nm.

[0102] The examples of hard inorganic powders having a Mohs' hardness offrom 5 to 9 include, e.g., α-iron oxide, α-alumina, and chromium oxide(Cr₂O₃). These powders may be used alone or in combination. Of the abovehard inorganic powders, α-iron oxide and α-alumina are preferred. Thecontent of hard inorganic powders in the backing layer is generally from3 to 30 mass parts, preferably from 3 to 20 mass parts, per 100 massparts of the carbon black.

[0103] When the above soft inorganic powder and hard inorganic powderare used in combination in the backing layer, it is preferred to usethem selectively so that the difference of hardness between soft andhard inorganic powders is 2 or more, more preferably 2.5 or more, andparticularly preferably 3 or more.

[0104] It is preferred that the above-described two kinds of inorganicpowders each having a specific average particle size and different inMohs' hardness and the above-described two kinds of carbon blacks eachhaving a different average particle size are contained in the backinglayer.

[0105] The backing layer may contain a lubricant. The lubricant can bearbitrarily selected from among those which can be used in the magneticlayer or the nonmagnetic layer as described above. The content of thelubricant in the backing layer is generally from 1 to 5 mass parts per100 mass parts of the binder.

[0106] Support

[0107] The supports which are used in the present invention arepreferably nonmagnetic flexible supports, and essentially have a thermalshrinkage factor of preferably 0.5% or less at 100° C. for 30 minutes,and preferably 0.5% or less at 80° C. for 30 minutes, more preferably0.2% or less, in every direction of in-plane of the support. Moreover,the above-described thermal shrinkage factors of the supports at 100° C.for 30 minutes and at 80° C. for 30 minutes are preferably almost equalin every direction of in-plane of the support with difference of notmore than 10%. The supports are preferably nonmagnetic supports. As thenonmagnetic supports which can be used in the present invention,well-known films such as polyesters (e.g., polyethylene terephthalateand polyethylene naphthalate), polyolefins, cellulose triacetate,polycarbonate, aromatic or aliphatic polyamide, polyimide,polyamideimide, polysulfone, and polybenzoxazole can be used. Highlystrong supports such as polyethylene naphthalate and polyamide arepreferably used. If necessary, a lamination type support as disclosed inJP-A-3-224127 can be used to vary the surface roughness of the magneticlayer surface and the base surface. These supports may be previouslysubjected to surface treatments, such as corona discharge treatment,plasma treatment, easy adhesion treatment, heat treatment, and dustremoving treatment. Aluminum or glass substrate can also be used as asupport in the present invention.

[0108] For attaining the object of the present invention, it ispreferred to use a support having a central plane average surfaceroughness (Ra) of 4.0 nm or less, preferably 2.0 nm or less, measured bya surface roughness meter TOPO-3D (a product of WYKO Co.). It ispreferred that the support not only has a small central plane averagesurface roughness but also is free from coarse protrusions having aheight of 0.5 μm or more. Surface roughness configuration is freelycontrolled by the size and the amount of the fillers added to thesupport. The examples of such fillers include acryl-based organicpowders, as well as oxides or carbonates of Ca, Si and Ti. The supportfor use in the present invention preferably has maximum height (Rmax) of1 μm or less, ten point average roughness (Rz) of 0.5 μm or less,central plane peak height (Rp) of 0.5 μm or less, central plane valleydepth (Rv) of 0.5 μm or less, central plane area factor (Sr) of from 10%to 90%, and average wavelength (λa) of from 5 μm to 300 μm. Forobtaining desired electromagnetic characteristics and durability,surface protrusion distribution of the support can be controlledarbitrarily by fillers, e.g., the number of protrusions having sizes offrom 0.01 μm to 1 μm can be controlled each within the range of from 0to 2,000 per 0.1 mm².

[0109] The F-5 value of the support for use in the present invention ispreferably from 5 to 50 kg/mm² (=about 49 to 490 MPa), the thermalshrinkage factor of the support at 100° C. for 30 minutes is preferably3% or less, more preferably 1.5% or less, and the thermal shrinkagefactor at 80° C. for 30 minutes is preferably 1% or less, morepreferably 0.5% or less. The support has a breaking strength of from 5to 100 kg/mm² (=about 49 to 980 MPa), an elastic modulus of from 100 to2,000 kg/mm² (=about 0.98 to 19.6 GPa), a temperature expansioncoefficient of from 10⁻⁴ to 10⁻⁸/° C., preferably from 10⁻⁵ to 10⁻⁶/°C., and a humidity expansion coefficient of 10⁻⁴/RH % or less,preferably 10⁻⁵/RH % or less. These thermal characteristics, dimensionalcharacteristics and mechanical strength characteristics are preferablyalmost equal in every direction of in-plane of the support withdifference of not more than 10%.

[0110] Producing Method

[0111] Producing process of the magnetic coating solution for themagnetic recording medium of the present invention comprises at least akneading step, a dispersing step and, optionally, blending steps to becarried out before and/or after the kneading and dispersing steps. Eachof these steps may be composed of two or more separate stages. All ofthe feedstocks such as a magnetic powder, a nonmagnetic powder, abinder, a carbon black, an abrasive, an antistatic agent, a lubricant,and a solvent, for use in the present invention may be added at any stepat anytime. Each feed stock may be added at two or more steps dividedly.For example, polyurethane can be added dividedly at a kneading step,adispersing step, or ablending step for adjusting viscosity afterdispersion. For achieving the object of the present invention,conventionally well-known techniques can be performed partly with theabove steps. Powerful kneading machines such as an open kneader, acontinuous kneader, a pressure kneader or an extruder are preferablyused in a kneading step. When a kneader is used, all or a part of thebinder (preferably 30% or more of the total binders) is kneading-treatedin the range of from 15 parts to 500 parts per 100 parts of the magneticpowder or nonmagnetic powder together with the magnetic powder or thenonmagnetic powder. These kneading are disclosed in detail inJP-A-1-106338 and JP-A-1-79274. For dispersing a magnetic layer solutionand a nonmagnetic layer solution, glass beads can be used, anddispersing media having a high specific gravity, e.g., zirconia beads,titania beads and steel beads are preferred for this purpose. Optimalparticle size and packing density of these dispersing media should beselected. Well-known dispersing apparatus can be used in the presentinvention.

[0112] For realizing the constitution of the present invention,successive multilayer coating of coating a lower layer and then coatinga magnetic layer after the lower layer has been dried is used. Awell-known successive multilayer coating method, i.e., a wet-on-drycoating method, is used, e.g., the methods disclosed in JP-A-3-214417and JP-A-3-214422 are preferably used.

[0113] The magnetic recording medium is subjected to orientationtreatment after coating, if necessary.

[0114] In the case of a magnetic disc, isotropic orienting property canbe sufficiently obtained in some cases without performing orientationusing orientating apparatus, but it is preferred to use well-knownrandom orientation apparatuses, to thereby dispose cobalt magnetsdiagonally and alternately or apply an alternating current magneticfield using a solenoid. Hexagonal ferrites in general have aninclination for three dimensional random orientation of in-plane and inthe vertical direction but it is also possible to make in-plane twodimensional random orientation. Further, it is also possible to impartisotropic magnetic characteristics in the circumferential direction byvertical orientation using well-known methods using, e.g., differentpole and counter position magnets. Vertical orientation is particularlypreferred when the disc is used in high density recording. Spin coatingmay be used for circumferential orientation.

[0115] In the case of a magnetic tape, orientation is effected in themachine direction by a cobalt magnet and a solenoid. In orientation, itis preferred that the drying position of the coated film can becontrolled by controlling the temperature and the amount of drying airand coating rate. Coating rate is preferably from 20 to 1,000 m/min. andthe temperature of drying air is preferably 60° C. or more. Preliminarydrying can be performed appropriately before entering the magnet zone.

[0116] Heat resisting plastic rolls, e.g., epoxy, polyimide, polyamideand polyimideamide or metal rolls are used for calendering treatment.Metal rolls are preferably used for the treatment particularly whenmagnetic layers are coated on both surfaces of a support. The treatmenttemperature is preferably 50° C. or more, more preferably 100° C. ormore. The linear pressure is preferably 200 kg/cm (=about 196 kN/m) ormore, more preferably 300 kg/cm (=about 294 kN/m) or more.

[0117] Physical Properties

[0118] The residual magnetic flux density×magnetic layer thickness inthe present invention is preferably from 5 to 200 mT·μm. The coerciveforce (Hc) is preferably from 1,800 to 5,000 Oe (=about 144 to 400kA/m), preferably from 1,800 to 3,000 Oe (=about 144 to 240 kA/m). Thedistribution of the coercive force is preferably narrow, and SFD(switching field distribution) and SFDr are preferably 0.6 or less.

[0119] The squareness ratio of a magnetic disc is from 0.55 to 0.67,preferably from 0.58 to 0.64, in the case of two dimensional randomorientation, from 0.45 to 0.55 in the case of three dimensional randomorientation, and in the case of vertical orientation, generally 0.6 ormore in the vertical direction, preferably 0.7 or more, and whendiamagnetic correction is performed, 0.7 or more, preferably 0.8 ormore. Degree of orientation of two dimensional random orientation andthree dimensional random orientation is preferably 0.8 or more. In thecase of two dimensional random orientation, the squareness ratio in thevertical direction, the Br in the vertical direction, and the Hc in thevertical direction are preferably from 0.1 to 0.5 times as small asthose in the in-plane direction.

[0120] In the case of a magnetic tape, the squareness ratio is 0.7 ormore, preferably 0.8 or more.

[0121] The magnetic recording medium in the present invention has afriction coefficient against a head at temperature of −10° C. to 40° C.and humidity of 0% to 95% of 0.5 or less, preferably 0.3 or less, theintrinsic resistivity of the magnetic layer surface of preferably from10⁴ to 10¹² Ω/sq, and the charge potential of preferably from −500 V to+500 V. The elastic modulus at 0.5% elongation of the magnetic layer ispreferably from 100 to 2,000 kg/mm² (=about 980 to 19,600 MPa) in everydirection of in-plane, the breaking strength is preferably from 10 to 70kg/mm² (=about 98 to 686 MPa), the elastic modulus of the magneticrecording medium is preferably from 100 to 1,500 kg/mm² (=about 980 to14,700 MPa) in every direction of in-plane, the residual elongation ispreferably 0.5% or less, and the thermal shrinkage factor at everytemperature of 100° C. or less is preferably 1% or less, more preferably0.5% or less, and most preferably 0.1% or less. The glass transitiontemperature of the magnetic layer (the maximum point of the loss elasticmodulus by dynamic visco-elasticity measurement at 110 Hz) is preferablyfrom 50° C. to 120° C., and that of the lower layer is preferably from0° C. to 100° C. The loss elastic modulus is preferably in the range offrom 1×10⁷ to 8×10⁸ Pa, and loss tangent is preferably 0.2 or less. Ifloss tangent is too large, adhesion failure is liable to occur. Thesethermal and mechanical characteristics are preferably almost equal inevery direction of in-plane of the medium with difference of not morethan 10%. The residual amount of a solvent in the magnetic layer ispreferably 100 mg/m² or less, more preferably 10 mg/m² or less. The voidratio of the coated layer is preferably 30% by volume or less, morepreferably 20% by volume or less, with both of the lower layer and theupper layer. The void ratio is preferably smaller for obtaining highoutput but in some cases a specific value should be preferably secureddepending on purposes. For example, in a disc-like medium which isrepeatedly used, large void ratio contributes to good running durabilityin many cases.

[0122] The magnetic layer has a central plane average surface roughness(Ra) measured by a surface roughness meter TOPO-3D (a product of WYKOCo.) of preferably 5.0 nm or less, more preferably 4.0 nm or less, andespecially preferably 3.5 nm or less. The magnetic layer preferably hasmaximum height (Rmax) of 0.5 μm or less, ten point average roughness(Rz) of 0.3 μm or less, central plane peak height (Rp) of 0.3 μm orless, central plane valley depth (Rv) of 0.3 μm or less, central planearea factor (Sr) of from 20% to 80%, and average wavelength (λa) of from5 μm to 300 μm. The surface protrusions of the magnetic layer of sizesof from 0.01 μm to 1 μm can be controlled arbitrarily within the rangeof from 0 to 2,000, and it is preferred to optimize electromagneticcharacteristics and a friction coefficient by controlling the surfaceprotrusions of the magnetic layer. The surface protrusions can be easilycontrolled by the control of the surface property of the support byfillers, the particle size and the amount of the magnetic powders addedto the magnetic layer, or by the surface shapes of the rolls of calendertreatment. The range of curling is preferably within ±3 mm. It can beeasily presumed that these physical properties of the magnetic recordingmedium in the present invention can be varied according to purposes inthe lower layer and the upper layer. For example, the elastic modulus ofthe upper layer is made higher to improve running durability and at thesame time the elastic modulus of the lower layer is made lower than thatof the upper layer to improve the head touching of the magneticrecording medium.

EXAMPLE

[0123] The present invention will be described in detail below withreference to the specific examples, but the present invention should notbe construed as being limited thereto. In the examples, “part” means“mass part (i.e., weight part)” unless otherwise indicated.

[0124] Sample 1

[0125] Each composition of Magnetic Coating Solution A and NonmagneticCoating Solution shown below was blended in a kneader, and dispersedwith a sand mill. Polyisocyanate was added to each resulting dispersionsolution, in an amount of 13 parts to Nonmagnetic Coating Solution and 4parts to Magnetic Coating Solution A, and 30 parts of cyclohexanone wasfurther added to each solution. Each solution was filtered through afilter having an average pore diameter of 1 μm to obtain coatingsolutions for forming a nonmagnetic layer and a magnetic layer.

[0126] The thus-obtained nonmagnetic coating solution was coated on apolyethylene terephthalate support having a thickness of 62 μm and acentral plane average surface roughness of 3 nm in a dry thickness of1.5 μm, and after drying the nonmagnetic layer (W/D coating), a magneticlayer was multilayer-coated in a thickness of 0.1 μm. After drying, thecoated layers were subjected to calendering treatment with calenders of7 stages at 90° C. at linear pressure of 300 kg/cm (294 kN/c). Theobtained web was punched to a disc of 3.7 inches, and the disc wassubjected to surface treatment by abrasives.

[0127] Sample 2

[0128] A magnetic layer coating solution and a nonmagnetic layer coatingsolution were prepared in the same manner as in the preparation ofSample 1. The obtained coating solutions were simultaneouslymultilayer-coated (W/W coating) on a polyethylene terephthalate supporthaving a thickness of 62 μm and a central plane average surfaceroughness of 3 nm. The nonmagnetic layer coating solution was coated ina dry thickness of 1.5 μm, immediately after that the magnetic layercoating solution was coated on the nonmagnetic layer so as to obtain themagnetic layer having a thickness of 0.1 μm. The coated layers weresubjected to random orientation while the magnetic layer and thenonmagnetic layer were still wet. After drying, the coated layers weresubjected to calendering treatment with calenders of 7 stages at 90° C.at linear pressure of 300 kg/cm. The obtained web was punched to a discof 3.7 inches, and the disc was subjected to surface treatment byabrasives.

[0129] Samples 3 to 12 and 21 to 28

[0130] Samples were prepared in the same manner as in the preparation ofSample 1 except that the average particle size and the addition amountof the diamond particles added to each magnetic layer coating solutionwere changed as shown in Table 1 below.

[0131] Sample 13

[0132] A sample was prepared in the same manner as in the preparation ofSample 1 except for using Magnetic Coating Solution B in place ofMagnetic Coating Solution A.

[0133] Sample 14

[0134] A sample was prepared in the same manner as in the preparation ofSample 2 except for using Magnetic Coating Solution B in place ofMagnetic Coating Solution A.

[0135] Sample 15

[0136] A sample was prepared in the same manner as in the preparation ofSample 1 except for changing the magnetic layer thickness from 0.1 μm to0.15 μm.

[0137] Sample 16

[0138] A sample was prepared in the same manner as in the preparation ofSample 1 except for changing the magnetic layer thickness from 0.1 μm to0.2 μm.

[0139] Sample 17

[0140] A sample was prepared in the same manner as in the preparation ofSample 2 except for changing the magnetic layer thickness from 0.1 μm to0.2 μm.

[0141] Sample 18

[0142] A sample was prepared in the same manner as in the preparation ofSample 1 except for reducing the time of dispersion of the nonmagneticcoating solution by a sand mill to one half.

[0143] Sample 19

[0144] A sample was prepared in the same manner as in the preparation ofSample 1 except for reducing the time of dispersion of the nonmagneticcoating solution by a sand mill to one third.

[0145] Sample 20

[0146] A sample was prepared by coating the nonmagnetic coating solutionused in Sample 1, drying, subjecting the coated layer to calenderingtreatment with calenders of 7 stages at 90° C. at linear pressure of 300kg/cm, then multilayer-coating a magnetic layer in a thickness of 0.1μm, drying, and again subjecting the coated layers to calenderingtreatment with calenders of 7 stages at 90° C. at linear pressure of 300kg/cm (294 kN/c). The obtained web was punched to a disc of 3.7 inches,and the disc was subjected to surface treatment by abrasives.Preparation of Coating Solution Magnetic Coating Solution AFerromagnetic metal fine powder 100 parts Composition: Fe 70 atomic %,Co 30 atomic % Hc: 2,300 Oe (184 kA/m) Average long axis length: 0.06 μmSpecific surface area: 55 m²/g Crystallite size: 115 Å σ_(s): 110 A ·m²/kg Sintering inhibitor: Al compound (Al/Fe, 14 atomic %) Y compound(Y/Fe, 7 atomic %) Vinyl chloride polymer  10 parts MR110 (manufacturedby Nippon Zeon Co., Ltd.) Polyurethane resin  4 parts UR 8200(manufactured by Toyobo Co., Ltd.) α-Alumina  5 parts Average particlesize: 0.15 μm Diamond particles shown in Table 1 (average particle sizeis shown in Table 1) Carbon black  1 part #50 (manufactured by AsahiCarbon Co., Ltd.) Phenylphosphonic acid  3 parts n-Butyl stearate  1part Butoxyethyl stearate  1 part Isocetyl stearate  2 parts Stearicacid  1 part Oleic acid  1 part Methyl ethyl ketone 140 partsCyclohexanone 200 parts Magnetic Coating Solution B Hexagonal bariumferrite 100 parts Surface treatment: Al₂O₅ 5 mass %, SiO₂ 2 mass % Hc:2,500 Oe (200 kA/m) Average tabular diameter: 30 nm Tabular ratio: 3σ_(s): 56 A · m²/kg Vinyl chloride copolymer  6 parts MR110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin  3 parts UR8200 (manufactured by Toyobo Co., Ltd.) α-Alumina  4 parts HIT60(manufactured by Sumitomo Chemical Co., Ltd.) Diamond particles shown inTable 1 (average particle size is shown in Table 1) Carbon black  1 part#50 (manufactured by Asahi Carbon Co., Ltd.) Isocetyl stearate  5 partsStearic acid  1 part Oleic acid  1 part Methyl ethyl ketone  80 partsCyclohexanone 120 parts Nonmagnetic Coating Solution (lower layer)Nonmagnetic powder: α-Fe₂O₃ 100 parts Average long axis length: 0.07 μmAverage short axis length: 0.014 μm Specific surface area (S_(BET)): 55m²/g pH: 9 Surface-covering compound: Al₂O₃, 8 mass % based on the totalparticles Carbon black  25 parts CONDUCTEX SC-U (manufactured byColumbia Carbon Co., Ltd.) Vinyl chloride copolymer  15 parts MR110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin  7 parts UR5500 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid  4 partsIsocetyl stearate  6 parts Oleic acid  1.3 parts Stearic acid  1.3 partsMethyl ethyl ketone/cyclohexanone 250 parts (8/2 mixture)

[0147] The performances of the above-obtained samples were evaluated asfollows. The results obtained are shown in Table 1 below.

[0148] S/N Ratio

[0149] S/N ratio was measured using RAW1001 type disc evaluatingapparatus (manufactured by GUZIK Co., U.S.A.), a spin stand LS-90(manufactured by Kyodo Electron System Co., Ltd.), and a metal-in-gaphead having a gap length of 0.2 μm. Writing of signals of line recordingdensity 100 KFCI was performed at the position of radius of 24.6 mm, andthe recorded signals were reproduced using an MR head having a trackwidth of 2.6 μm. S/N ratio was obtained from the reproduction output(TAA) and the noise level after DC erasure of the disc.

[0150] PW50 (Half Value Breadth of Isolated Pulse (Wave) Shape)

[0151] PW50 was measured using RWA1001 type disc evaluating apparatus(manufactured by GUZIK Co., U.S.A.), a spin stand LS-90 (manufactured byKyodo Electron System Co., Ltd.), and a metal-in-gap head having a trackwidth of 5 μm and a gap length of 0.2 μm. PW50 was obtained byperforming writing of signals of isolated pulse (wave) shape at theposition of radius of 24.6 mm, and reproducing the recorded signalsusing an MR head having track width of 2.6 μm.

[0152] Evaluation of Fluctuation of Magnetic Layer Thickness by EPMA

[0153] Intensity mapping of the objective element in the range of100×100 μm from the magnetic layer surface was performed by at least500×500 pixel on conditions of an electron beam accelerating voltage of15 kV, 30 nA, and a beam diameter of 1 μmφ using EPMA-1600 manufacturedby Shimadzu Seisakusho Co. In the case of ferromagnetic metal powders,Co was selected as the inherent element of the magnetic layer, and inthe case of hexagonal ferrite magnetic powder, Ba was selected as theinherent element of the magnetic layer.

[0154] In the result of the obtained intensity mapping of element, theintensity was divided to 256 stages, and the standard deviation ofintensity distribution (b) and the average value (a) were obtained withan image analyzer KS400 manufactured by Zeiss Corp., and b/a wascomputed.

[0155] Evaluation of Running Durability

[0156] A floppy disc drive (ZIP100, a product of IOMEGA CORP., U.S.A.,rotation number: 2,968 rpm) was used and the head was fixed at theposition of radius of 38 mm. Recording was performed with recordingdensity of 34 kfci, and the recorded signals were reproduced and thiswas taken as 100%. The disc was run under the thermo-cycle condition for1,500 hours with the flow shown in Table 2 below as one cycle. Outputwas monitored every 24 hours of running and the point when the outputbecame 70% or less of the initial value was taken as NG. TABLE 1 DiamondMagnetic Average X Magnetic Addition Sample Coating Particle LayerAmount No. Remarks Solution Size (μm) Thickness* (parts)  1 Invention A0.1 1 1  2 Comparison A 0.1 1 1  3 Invention A 0.2 2 1  4 Comparison A0.3 3 1  5 Invention A 0.02 0.2 1  6 Comparison A 0.01 0.1 1  7Invention A 0.1 1 3  8 Invention A 0.1 1 5  9 Comparison A 0.1 1 10 10Invention A 0.1 1 0.5 11 Invention A 0.1 1 0.1 12 Comparison A 0.1 10.05 13 Invention B 0.1 1 1 14 Comparison B 0.1 1 1 15 Invention A 0.10.67 1 16 Comparison A 0.1 0.5 1 17 Comparison A 0.1 0.5 1 18 InventionA 0.1 1 1 19 Invention A 0.1 1 1 20 Invention A 0.1 1 1 21 Comparison A0.02 0.2 0.05 22 Invention A 0.02 0.2 0.1 23 Invention A 0.02 0.2 5 24Comparison A 0.02 0.2 8 25 Comparison A 0.2 2 0.05 26 Invention A 0.2 20.1 27 Invention A 0.2 2 5 28 Comparison A 0.2 2 8 Mag- Duration neticof life Layer Lower (hr) Sam- Thick- Layer b/a (Running ple ness CoatingRa (% S/N PW50 Dura- No. (μm) Method (nm) ) (dB) (μm) bility)  1 0.1wet-on-dry 10 20 28 0.3 1,500<  2 0.1 wet-on-wet — 50 20 0.4 1,500<  30.1 wet-on-dry 10 25 27 0.3 1,500<  4 0.1 wet-on-dry 10 40 22 0.3   800 5 0.1 wet-on-dry 10 18 29 0.3 1,500<  6 0.1 wet-on-dry 10 17 29 0.3  500  7 0.1 wet-on-dry 10 22 27 0.3 1,500<  8 0.1 wet-on-dry 10 25 260.3 1,500<  9 0.1 wet-on-dry 10 40 22 0.3   500 10 0.1 wet-on-dry 10 1829 0.3 1,500< 11 0.1 wet-on-dry 10 17 29 0.3 1,200< 12 0.1 wet-on-dry 1017 30 0.3   500 13 0.1 wet-on-dry 10 20 32 0.3 1,500< 14 0.1 wet-on-wet— 50 24 0.3 1,500< 15 0.15 wet-on-dry 10 15 27 0.32 1,500< 16 0.2wet-on-dry 10 10 24 0.45 1,500< 17 0.2 wet-on-wet — 30 22 0.5 1,500< 180.1 wet-on-dry 20 25 27 0.3 1,500< 19 0.1 wet-on-dry 30 30 24 0.3 1,500<20 0.1 wet-on-dry  5 10 29 0.3 1,500< 21 0.1 wet-on-dy 10 18 30 0.3  300 22 0.1 Wet-on-dry 10 18 30 0.3 1,000 23 0.1 Wet-on-dry 10 18 280.3 1,500< 24 0.1 Wet-on-dry 10 18 23 0.3   800 25 0.1 Wet-on-dry 10 2528 0.3   700 26 0.1 Wet-on-dry 10 25 28 0.3 1,500< 27 0.1 Wet-on-dry 1025 26 0.3 1,000 28 0.1 Wet-on-dry 10 25 20 0.3   300

[0157] TABLE 2

[0158] As is apparent from the results shown in Table 1, the samplesaccording to the present invention are excellent in all of S/N, PW50 anddurability, however, the comparative samples are inferior to the samplesof the present invention in any of these properties.

Effect of the Invention

[0159] It is essential to heighten track density for increasing thecapacity of magnetic recording media, however, it becomes necessary toadopt an MR head with the thinning of a track width. The MR head hashigh sensitivity, but is susceptible to medium noise, and microscopicthickness dispersion of a magnetic layer causes noises, and thistendency is conspicuous when a magnetic layer thickness is as thin as0.15 μm or less. It has been found that wet-on-dry (W/D) coating iseffective for reducing noises attributable to the distribution of amagnetic layer thickness.

[0160] On the other hand, so far it has not been possible to securesufficient durability with the samples formed by W/D coating, butaccording to the present invention sufficient durability could beobtained by adding diamond particles having an average particle size offrom ⅕ to 2 times the thickness of a magnetic layer to the magneticlayer in an amount of from 0.1 to 5.0 mass % based on the amount of theferromagnetic powder.

[0161] The entitle disclosure of each and every foreign patentapplication from which the benefit of foreign priority has been claimedin the present application is incorporated herein by reference, as iffully set forth herein.

[0162] While the invention has been described in detail and withreference to specific examples thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A magnetic recording medium comprising a supporthaving provided thereon a substantially nonmagnetic lower layer bycoating a lower layer coating solution comprising a nonmagnetic powderdispersed in a binder and drying, and a magnetic layer having athickness of from 0.01 to 0.15 μm by coating a magnetic layer coatingsolution comprising a ferromagnetic metal powder or a hexagonal ferritepowder dispersed in a binder, wherein the magnetic layer containsdiamond particles having an average particle size of from ⅕ to 2 timesthe thickness of the magnetic layer in an amount of from 0.1 to 5.0 mass% based on the ferromagnetic metal powder.
 2. The magnetic recordingmedium as in claim 1, wherein the lower layer has an average surfaceroughness of 20 nm or less.
 3. The magnetic recording medium which isfor reproduction using an MR head.